The fabrication of integrated circuits typically involves transferring circuit patterns from a photolithographic mask or reticle onto a photosensitive resist layer on a substrate, such as a semiconductor substrate. As the terms are typically used, a mask may be somewhat different than a reticle. A mask, for example, typically includes a pattern that covers an entire substrate, while a reticle includes a pattern that covers only a few devices, and which is stepped across the surface of the substrate to pattern the entire substrate. In addition, a mask is typically a one to one representation of the images to be formed, while a reticle may be a larger representation of the images to be formed, such as a four to one representation, which is then optically reduced in size during the exposure process. However, as used herein, the term “mask” is intended to include all such imaging structures, whether they be masks or reticles.
After a pattern from the mask has been defined in the resist layer, a process is performed on the substrate, such as etching or doping an underlying layer, or depositing a new layer. Once this has been accomplished, the layer of resist is removed. Multiple different patterns are repeatedly transferred onto the substrate in this general manner, with each step typically using a different mask or mask to form the separately patterned layers. At the end of the fabrication process the substrate is singulated into a plurality of dice for subsequent packaging as separate integrated circuits.
There are a few different methods by which a mask can be formed. The fabrication process generally begins with data corresponding to the circuit pattern, which is typically a representational layout of the physical layers of the integrated circuit. However, there are different methods by which the data is transferred to a physical representation of an image on the mask substrate. One general method is by directly writing the image onto the mask substrate, and another general method is by using an intermediate imaging technique, such as a photo repeater. Each method has its benefits and drawbacks.
A mask writer typically uses an imaging system such as a laser scanner or an electron beam writer. With this method, the image data is written directly onto the mask substrate, such as burning away an opaque layer on the transparent mask substrate, depositing an opaque layer in a pattern on the mask substrate, or directly exposing a pattern into a photoresist layer on the mask substrate. Regardless of the specific method used, direct writing of the desired pattern is accomplished without any type of intermediate structure. Direct writing of the mask is a good method to be used when, for example, the pattern to be formed on the mask is highly customized. By direct writing such a customized pattern, other intermediate imaging structures do not need to be formed as a part of the mask fabrication process. Typically, such intermediate structures cost a lot to produce. Since the intermediate structures for a highly customized pattern would not be widely used thereafter, their cost would have to be entirely absorbed by the few masks in which they were used. Thus, direct writing of a mask may tend to reduce the cost of the mask in certain circumstances.
The other general method of mask fabrication uses a master mask, which is an intermediate structure used to pattern the mask being fabricated. Typically, patterns on the master mask are formed at a larger size, and are then reduced during the exposure process onto the mask being formed. Thus, the patterns on the master mask are repeated as many times as desired onto the mask being fabricated. Mask repeaters are typically used to fabricate masks that include standard device patterns, such as patterns for central processing units, random access memory, read only memory, digital signal processors, digital to analog converters, and other standard designs that are shared by devices such as system on chip and memory. Because the patterns for such functional units are typically used again and again in various integrated circuit designs, the cost of the intermediate structures is spread across many different mask sets. Thus, photo repeating tends to reduce the cost of certain masks formed with this method.
In some instances, both direct writing and photo repeating are used to form a mask. For example, some portions of the integrated circuit pattern of a given layer may be formed by direct writing of the pattern, and other portions of the integrated circuit pattern of the given layer may be formed by photo repeating from a master mask. Specifically, the patterns for standard functional units may be formed on the mask using photo repeating, while more customized functional units on the mask may be formed using direct writing.
Once fabricated, the mask is inspected for manufacturing defects. Inspection of the mask is typically accomplished using an automated inspection apparatus. Typically optical images of the mask structures are compared to baseline images. The baseline image may be generated from the circuit pattern data, from another mask, or from an adjacent corresponding image on the mask being inspected. During comparison of the images, any difference between the inspected structure and the baseline structure is compared to a threshold value, with any difference in excess of the threshold indicating a defect.
The magnification at which the inspection is conducted influences how many discrepancies are flagged between the inspected structure and the baseline structure. At a higher magnification, or in other words at a high resolution, a greater number of discrepancies are typically found, while at a lower magnification, or in other words at a lower resolution, a fewer number of discrepancies are typically found. Thus, it is desirable at the onset of inspection to specify the resolution at which the inspection will be conducted.
However, mask patterns formed by different methods tend to have different optimal inspection resolutions. For example, patterns imaged using mask repeaters generally have improved pattern uniformity and relatively smooth line edges, which generally tolerate a relatively higher inspection resolution. On the other hand, patterns imaged using direct writing generally have relatively rougher line edges, which generally require a relatively lower inspection resolution. In addition, different functional units on a mask layer may have different optimal critical dimension criteria, regardless of the method by which they are imaged on the mask, and thus would most preferably be inspected at different resolutions. Unfortunately, inspection equipment does not have the flexibility to make inspections at variable resolutions based upon such criteria.
What is needed, therefore, is a system for inspecting a mask using different resolutions that are tailored to the properties of the patterns being inspected.